An access point (AP) generates a multi-user request to send (MU-RTS) frame for protecting a channel bandwidth during a communication exchange, and transmits the MU-RTS frame in multiple duplicate first packets in respective communication subchannels that span the channel bandwidth. Each first packet has a first packet format that conforms to a legacy protocol. The AP receives multiple clear-to-send (CTS) frames from multiple client stations via respective communication subchannels that span the channel bandwidth. The AP generates a trigger frame configured to prompt the multiple client stations to transmit respective data to the AP. After transmitting the multiple first packets, the AP transmits the trigger frame in one or more second packets that span the channel bandwidth and that conform to a second packet format defined by a non-legacy protocol. The AP receives, from the multiple client stations, data frames that are responsive to the trigger frame.